Zircon U–Pb ages and Hf isotopic analyses of migmatite from the ‘paired metamorphic belt’ in Chinese SW Tianshan: Constraints on partial melting associated with orogeny

• Zircon U–Pb ages and Hf isotopes were studied on migmatites in Chinese SW Tianshan.
• Polyphase anatexis occurred while the dominant phase happened at ~ 400 Ma.
• Hf sources for new zircons were from pre-existing zircons in a close system.
• The crustal formation of Yili and Central Tianshan mostly occurred in Proterozoic times

A paired metamorphic belt with ubiquitous migmatites and rare granulites in its high dT/dP part formed due to oceanic subduction in Chinese southwestern (SW) Tianshan. Although several geochronological studies have addressed it in recent years, the exact timing of the high-grade metamorphism and partial melting is still controversial. For this study, we selected nine samples obtained from three valleys transecting the SW Tianshan Migmatite Complex for U–Pb geochronology and Lu–Hf isotope analysis to provide constraints on partial melting and the mid-crustal evolution during oceanic subduction and subsequent continental collision.On the basis of internal morphology, Th/U ratios and REE patterns, three types of zircons are discriminated. Dating them reveals five age groups: (1) Inherited zircons either are detrital, with ages scattering from 432 Ma to 3261 Ma, or yield ages concentrated at 406–410 Ma reflecting Early Devonian magmatism; (2) Recrystallized zircon has been transformed from pre-existing zircon under subsolidus conditions at ~ 400 Ma and ~ 360 Ma; (3) Metamorphic rims have grown on zircons at ~ 400 Ma, ~ 290 Ma and ~ 270 Ma, corresponding to three phases of anatexis. The partial melting phase at ~ 400 Ma is probably related to the emplacement of voluminous mafic to felsic magmas in a continental arc region above the subduction zone consuming the South Tianshan Paleo-Ocean that triggered the most extensive regional-scale thermal event. However, the two subsequent partial melting phases at ~ 290 Ma and ~ 270 Ma representing post-collisional stages were not penetrative. P–T conditions of the studied migmatites estimated by Hb–Pl thermobarometry and Ti-in-zircon thermometry are 682–763 °C, 4.7–8.0 kbar; together with composite Kfs + Q + Pl, Kfs + Q and Kfs + Q + Ab inclusions representing entrapped melt and Kfs inclusions with Ab exsolution lamellae in zircon rims, the P–T estimates support the occurrence of anatexis. Zircon Lu–Hf isotope analysis indicates that zircon rims formed under high-grade metamorphic conditions and during partial melting by dissolution–precipitation of pre-existing zircons in a close system. Zircon two-stage Hf model ages constrain the development of the protoliths of the migmatites at four major crustal formation periods (~ 1871 Ma, 1368–1426 Ma, 1099–1276 Ma and 743 Ma). Combined with previous researches in this area, we conclude that the main crustal formation of the Yili and Central Tianshan Plates has mostly occurred in Proterozoic times, but not or only subordinately during the Archean.